How to Do Compression and Leak Down Tests

Compression Test

If your machine just doesn’t have the snap that it use to have, you might have issues with the compression of the engine cylinders.  If your bike or sled won’t even start and all your electrical is working, you almost definitely could have some compression issues.  An engine needs enough compression to compress the fuel mixture, which raises the temperature enough to be combustible.  A simple compression test can tell you if you need to further investigate your engine to figure out the problem before serious damage can occur.

The initial cost of a compression tester might scare you off, but a shop bill or the cost of repairing unattended engine problems could cost much more.  Once you make the investment of buying the tester, you will always have it for future use.  Just make sure that you get it back from your buddy after he borrows it for his own machine.

Before you perform the compression test, you will want to start the machine (if it will start) and get it up to operating temperature.  By warming the engine up, it will expand the metal to ensure that you are getting the best seal to produce accurate operating compression.

Now that the engine is warm, you will want to turn the fuel off and run the machine until it runs out of fuel.  If your bike or sled is fuel injected, you will need to remove the fuse for the fuel pump to stop the fuel from being pumped in.  You will also need to remove the ignition fuse if your machine has one.  Next, you will want to remove all of the spark plugs from the engine.  With the plugs out, you can now install the compression tester.  Because there are different size spark plugs, most gauges will come with different size adapters.  It is important that you choose the right size adapter for your machine, otherwise the compression readings will be off.  When installing the tester, add some grease to the threads and O-ring.

Now that the compression tester is installed, you can now start the test.  With the on/off switch in the off position, you can begin turning the engine over.  It is important to keep the throttle wide open when you start turning over the engine.  Keep turning over the engine until the gauge on the tester stops going up.  When it reaches the maximum compression(the needle stops moving), record the results and then repeat the process for each cylinder.

Most engines will need at least 100psi to even start and run, but at that low of compression, it will not run very well.  The optimum range for compression is usually 120psi and above and each cylinder should be within 10% of one another.  If you have the owner’s manual for your bike or sled, it will tell you the exact psi range that your machine should be at.  When you have a psi reading that is 110psi or lower, you should probably investigate further.  One way to do that is by doing a Leak Down Test

Leakdown Test

The compression test will tell you that you have a problem, but the leak down test will lead you to where the problem actually is.  The basic premise of the test is to pressurize the cylinder and look and listen for leaks.  Possible leaks could be found at the piston rings, valve seats, crank seals or at the head gaskets.  The air leaks will not only create low compression, but they could also lean out the engine, which could lead to it burning down.

Four strokes and two strokes require different testers to be able to do the leak down test.  These tips are for using a four stroke tester.  Before you attach the leak down tester, you will want to get your machine back up to operating temps.  Once it is warmed up, remove the spark plugs and then you will need the piston of the cylinder that you want to test first at top dead center.  Now you can thread the tester on through the spark plug hole after applying grease or oil to the threads and O-ring.

Next, attach the leak down tester to your air compressor and then slowly turn the air up to 100psi.  If you do not have a compression tester, you can use this tool as a cheater for that as well.  If the cylinder pressure is at least 95%, it is ok.  Anything less and you will need to look for the leaks.  If you do not hear any leaking from the intake or exhaust ports, you will then be able to narrow it down to a bad head gasket or the piston rings.

You can check for a bad head gasket by applying soapy water all the way around the gasket where the head meets the cylinder.  If you can see bubbles being formed, that is where the leak is coming from.  Remember or take note of where the bubbles are so you will be able to locate the problem when you take the head off.

To check to see if it is the piston rings that are bad, you will need about an ounce of clean oil.  Remove the tester and add the oil to the cylinder.  Then reapply the tester.  If it reads 95% or above, you will then know that the piston rings are bad and need to be replaced.

Leak down testing needs to be done differently on a two stroke compared to a four stroke.  For a two stroke, you need to plug off the intake and exhaust of the engine.  Then you will only need to pump up it up to about 5psi, usually with a hand pump.  Anything above that and you run the risk of damaging the crank seals.  Once you reach 5psi, you can watch the gauge to see if you are losing pressure.  If you are losing pressure, you can spray soapy water around all of the gaskets, studs and seals and look for bubbles forming.

Two stroke leak down testers can be made by the average DIYer because of the low psi that is needed.  Because the four stroke tester requires a regulator to manage the higher psi, it usually is a good idea to buy one to get the most accurate readings.

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Ryan Swanson

Ryan is one of the lucky ones who gets to combine their passion with work. He has enjoyed powersports his whole life and now gets to write about it. Ryan has been around the industry since being a teenager, having worked in the Dennis Kirk store and warehouse while going to school. This has given him hands-on experience with the product DK carries and one-on-one relationships with customers.

He continues to enjoy learning and sharing about powersports with others in his role as a Web Content Analyst and writer for Dennis Kirk, a role he's been in since 2015. You'll find articles written by Ryan that range from product overviews, how-tos, tech tips, lifestyle articles, and more.

When he's not writing about powersports, Ryan is often enjoying them himself. It doesn't matter if the machine has four wheels, two wheels, or a track, he finds enjoyment in it all.

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